Remove tui_data_window::display_regs

Remove tui_data_window::display_regs

There's no need for tui_data_window::display_regs any more (if there
ever was).  All the paths through data window construction will end up
setting this to true.  This patch removes the member.

2019-08-20  Tom Tromey  <tom@tromey.com>

	* tui/tui-regs.h (struct tui_data_window) <display_regs>: Remove.
	* tui/tui-regs.c (tui_data_window::show_registers): Update.
	(tui_data_window::check_register_values): Update.